Method of manufacturing aluminium electrode foil for electrolyte capacitors

ABSTRACT

Enlargement of the effective surface of aluminum foil for electrolytic capacitors by electrolytical etching in the aqueous solution of 1-5 mol/l of an alkali halogenide and 1-5 mol/l boric acid at a pH of between 0 and 2.5. 2.

United States Patent 1 91 Van Herwijnen et al.

METHOD OF MANUFACTURING ALUMINIUM ELECTRODE FOIL FOR ELECTROLYTECAPACITORS inventors: Arend Van I-Ierwijnen; Pieter Marten Vogel, bothof Centuurbaan, Zwolle, Netherlands U.S. Philips Corporation, New York,N.Y.

Filed: Dec. 28, 1970 Appl. No.: 102,098

Assignee:

Foreign Application Priority Data Jan. 2, 1970 Netherlands 7000002 U.S.Cl. 204/l29.75, 204/l29.8 Int. Cl. C231) 1/00, C23b 3/02 Field of Search204/141, 129.75,

[451 Aug. 28, 1973 Primary Examiner-John H. Mack Assistant Examiner-T.Tufariello Attorney-Frank R. Trifari [57] ABSTRACT Enlargement of theeffective surface of aluminum foil for electrolytic capacitors byelectrolytical etching in the aqueous solution of 1-5 moi/l of an alkalihalogenide and 1-5 moi/1 boric acid at a pH of between 0 and 2.5. 2.5

5 Claims, No Drawings METHOD OF MANUFACTURING ALUMINIUM ELECTRODE FOILFOR ELECTROLYTE CAPACITORS The invention relates to a method ofmanufacturing aluminum electrode foil having an enlarged effectivesurface for use in electrolytic capacitors.

To enlarge the effective surface an aluminum foil is electrolyticallyetched. The capacitive surface enlargement obtained thereby is alsodependent on the formation voltage. As a rule a chloride-containingaqueous solution, for example, a sodium chloride solution is used as anelectrolyte solution. It is known that certain additions to thesechloride containing solutions may bring about a further enlargement ofthe effective surface, which results in an increase of the capacity persurface unit.

Thus, it is known, for example, from the United Kingdom Specification1,1 10,131 to add anions to the usual electrolyte solutions for etchingsuch as borate, phosphate, oxalate, nitrate or citrate ions. Suchadditions result in an enlargement of the effective surface by to 50percent.

It is also known from the United Kingdom Specification 1,181,556 that afurther increase of the etching factor, which is the term used toindicate the enlargement factor of the effective surface,-can beachieved by using hard foil as a basic product instead of the commonlyused soft annealed foil. However, this has practical drawbacks of amechanical nature, especially when using broad coils of foil. Afteretching the foil is rather brittle, stiff and has a low stretch. 1naddition the foil must be soft annealed after etching. By carrying outthe latter method in accordance with a method specified in this patentapplication a further increase of the etching factor is obtained.

The present invention provides an etching method in which theenlargement of the effective surface is such that the combined effect ofuse of the known additions and the choice of hard foils is equalledwhile yet starting from a soft foil.

The method according to the invention is characterized in that boricacid' is added to a concentration of l to 5 mol per litre to the etchingelectrolyte solution which consists of an aqueous solution of an alkalihalide likewise in a concentration of from 1 to 5 mol per litre, whilethe pH of the solution is maintained at a value of between 0 and 2.5.

The adjustment of the pH-value may be effected advantageously be meansof sulphuric acid, but hydrochloric acid may alternatively be used. ThepI-l-value is preferably maintained between 1 and 1.5.

Boric acid is present in the solution preferably in a concentration offrom 2.5 to 3.5 mol perlitre.

According to the first-mentioned patent application boric acid-is addedin a quantity of between 0.1 and 1 mol per litre to an etchingelectrolyte, while the pH is factor achieved thereby relative to themethod employing an electrolyte solution without an addition isconsiderably less than that according to the invention. Likewise as inthe known methods employing additions in the etching liquid, theaddition according to the invention results in a reduced mechanicalstretch of the foil obtained. Therefore also according to the invention,a preferred embodiment of the method employs a reduced quantity ofelectric charge as compared with the conventional quantity. This resultsin an increase of the tensile strength of the foil obtained so that itis suffciently strong for practical use as well as for machine formingand continuous forming.

The known step of a thermal post-treatment of the etched foil results ina still further enlargement of the effective capacitive surface of thefoil when the foil is heated at a temperature of between 400 and 650C inan oxygen-containing atmosphere. The period during which the foil mustbe thermally post-treated depends on the temperature chosen. If acertain maximum value of the period is exceeded, initially highcapacitance's become manifest which, however, are greatly decreased inan unreproducible manner when the foil is finished. An empiricalrelationship between the maximum heating duration t(ii'| seconds) andthe chosen temperature of treatment T (in K) was found:

logt=90.230logT The foil is preferably treated duringthe period whichcovers at least 24 percent of the maximum 'period of time found inaccordance with this formula.

in order that the invention may be readily carried into effect, it wilnow be described in detail by way of some examples.

Soft aluminum foil which was heated for 4 hours at 550C and which had athickness of 80 microns and a purity of 99.98 percent waselectrochemically=etched at a pulsatory direct current being smoothed bypercent in an aqueous solution of 100C containing per litre: 300 gms ofsodiumchloride and 100 gms of boric acid.

, Table I gives a survey of the physical values obtained from a foilwhich is etched in such an electrolyte liquid as comparedwith a foilwhich is etched in an electrolyte liquid consisting of a solution of 300gms per litre of sodium chloride only l DUring these experiments thefollowing values were varied: the electric charge in Coulombs per sq.dmof surface calculated on both sides, the current density (i) in A/sq.dmand the pH adjusted with l-lCl and with H 80 Comparative capacitancesare given for the foil which is formed at three different voltages.Finally the impact strength is stated, measured on 15 mm ,wide strips offoil expressed in cm.kg. In some cases comparative measured values arealso given for hard foil which is etched in the same manner. Table 11states some properties of the foil electrolytically etched in a bathmaintained neutral. The improvement of the etching led.

TABLE'I I Capacity in F/cmfi, formed at- Impact Electr. strength charge(W 50 V \cm. kg.)

(coul/ i (A/ sq. dm) dmfi). Hard Soft Hard Soft Hard. Soft Hard Softwhose composition is var- TA LE 1 unlilnlwl Capacity in F/cmfi, formedat- Impact Electr stmigth ln Pu charge 6\' 50V 90V (cn1.kg.) etchingadjusted (coull 101/ Number sol with sq.dm) dmfl) Hard Soft Hard SoftHard Soft Hard Soft 1.0 Hzsot 3,500 8.7 0. Q 1.5 Hzsot 3,500 s.s 0.502.0 H2804 3,500 8.0 0.35 2.5 H2804 3,500 7.1 0.51 3.0 H1801 3, 500 6.0.0. 84

TABLE 11 Capacity F/crnfl), formed at- Pn ad- Electr. 6V 50v 90V Na PHetchjusted charge Number g/lOOml H3130; ing sol. with (CJdmJ) Soft Hard200 1. H2504 3. 500 6.8 200 50 1. 5 H2801 a, 500 6.7 200 100 1. 5 H25013, 500 r. 5 200 150 1. 5 H280. 3, 500 8. 2 200 200 1. 5 H2804 3, 500 0.3 250 200 1. 5 H1304 3. 500 9. 6 300 200 1. 5 H2504 3, 500 .5 100 2001.0 H1801 3,500 8.0 150 200 1. 0 111504 3, 500 0. 0 200 200 1.0 11130.3,500 300 100 7.4 NaOlI 3,500 5.2 300 100 7.4 No.01! 4,500 0.5

We claim: tams borlc acid in an amount of between 2.5-3.5 mols l. Amethod of treating aluminum metal to increase per liter. the effectivesurface thereof, said method comprising 4. The method of claim 2 whereinafter etching the electrolytically etching said metal in an electrolyte50- metal is heated at a temperature of between 400 and lution having apH ofbetween 0 and 2.5 and containing 650C for a maximum period of timet in seconds rel-5 mols per liter of sodium chloride and l-5 mols perlated to the heating temperature T in according to liter of boric acid.the equation:

2. The method of claim 1 wherein the pH of the solulog t 90.2 30 log T.tion lies between 1 and 1.5. 5. An electrode foil manufactured by themethod of 3. The method of claim 2 wherein the solution conclaim 1.

o-w d UNITED STATES PATENT OFFICE 6 CERTIFICATE OF CORRECTION Pafie'otNo. 3,755,115 {PI-IN 4526) Doteci August 528, 1975 Inventofls) AREND VANHERWIJNEN ETAL .It is certifidftfxat errorappars in the above-identifiedpatent and that said Letters Patent are hereby correctd as shown below:7

Column 2, "line 29, 24 perceut should read 25 peroenf line 32, "wil"should read -will lino 46, "DUring Should 'rad Durincj Last line ofabstract: v I 5 -%2. 5.'2.5" should-read #4 "2 1.5; I I

Signed and sealed this 18th oay ofv December 19 73.

(SEAL) Attest: V EDWARD M. FLETCHER, JR. RENE D. TEGTT EYER AttestingOfficer Acting Commissioner of Patents

2. The method of claim 1 wherein the pH of the solution lies between 1and 1.5.
 3. The method of claim 2 wherein the solution contains boricacid in an amount of between 2.5-3.5 mols per liter.
 4. The method ofclaim 2 wherein after etching the metal is heated at a temperature ofbetween 400* and 650*C for a maximum period of time t in seconds relatedto the heating temperature T in *K according to the equation: log t90.2 - 30 log T.
 5. An electrode foil manufactured by the method ofclaim 1.